Leo Varadkar spoke out after Ms May surprised MPs by saying “we are looking” at arrangements at the crossing in North America, as she wrestles with how to avoid a hard Irish border.
The comment prompted mockery in the Commons, where Jenny Chapman, a Labour Brexit spokeswoman, said: “There are guns and armed customs guards on that border. Surely that is not what she has in mind?”
Speaking in Dublin, Mr Varadkar said he had not heard the UK Prime Minister’s statement to Parliament, but said the US-Canada border would “definitely” not be acceptable in Ireland.
“I visited the US-Canada border, I visited it back in August, and I saw a hard border with physical infrastructure with customs posts, people in uniforms with arms and dogs and that is definitely not a solution that is one that we can possibly entertain,” he said.
The criticism underlines how the future Irish border is among the most difficult issues in the Brexit negotiations, threatening to wreck the Prime Minister’s hopes for a deal.
In December, Ms May agreed to “full alignment” of regulations across the entire UK if necessary to avoid the return of border posts and checks, which could become a magnet for terrorists.
But she has been accused of reneging on that agreement, rejecting the EU’s proposal of a “common regulatory area" across Ireland if other solutions to avoid a hard border fail.
Instead, the Government is putting its faith in technology to avoid checks – while failing to rule out the return of cameras at the crossing.
Asked if he was frustrated by the UK Government, Mr Varadkar said: “No, no, I never get frustrated by the British and I'm always very patient.”
Updating MPs following her Mansion House speech on Brexit, Ms May said: “There are many examples of different arrangements for customs around the rest of the world.
“Indeed, we are looking at those, including for example the border between the United States and Canada.”
Challenged by Labour MPs, she appeared to backtrack, saying: “What I said was that we are looking at the border arrangements in a number of countries around the world” – but without mentioning the US.
Chris Leslie, a Labour supporter of the pro-EU Open Britain group, said: “The Government has repeatedly ruled out any return to a hard border on the island of Ireland, as well as any new physical infrastructure at the border.
“But anyone who's visited the US-Canada border will know that it's a very hard border indeed, with customs checks, barriers and armed guards.
“Leave campaigners told us there'd be “no change” to the border if we voted to leave the EU. That promise is looking less deliverable by the day.”
In the Commons, Ms May said: “As Prime Minister, I am not going to let our departure from the EU do anything to set back the historic progress made in Northern Ireland.
“Nor will I allow anything that would damage the integrity of our precious union.”
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